1. Field of the Invention
The present invention relates to a crank chamber communication structure of a multi-cylinder internal combustion engine, and particularly relates to a crank chamber communication structure of a multi-cylinder internal combustion engine in which a breather is formed between adjoining crank chambers in a crankcase.
2. Description of the Related Art
In a multi-cylinder internal combustion engine, the propulsive force of pistons that receive combustion pressure in cylinders is transmitted to the crankpins of a crankshaft corresponding to the respective cylinders, and the propulsive force is converted into rotation of the crankshaft. Therefore, the crankshaft is often supported by partition walls that are disposed between spaces in the crankcase that correspond to an adjoining cylinder, in other words, a crank chamber via bearings. Further, because pumping loss may be reduced when air (including air, blow-by gas, and so forth) is allowed to move between the adjoining cylinders while the pistons of the adjoining cylinders move in opposite directions, it is known that communicating holes are provided in the partition walls to allow movement of air.
In a crank chamber communication structure of a multi-cylinder internal combustion engine that has such a communicating hole, for example, a generally cylindrical cylinder inner wall in which the piston slides is formed separately from a cylinder block. Further, a protruding end is provided on the cylinder inner wall to protrude in the axial direction in a vicinity of the partition wall so that fastening force of a cylinder head to the cylinder block is transmitted to a section of the partition wall between the adjoining crank chambers, through which the crankshaft passes, as a compressive load via the cylinder inner wall. This prevents the formation of cracks and so forth in the crankshaft pass-through section of the partition wall. Such a communication structure is described, for example, in Japanese Patent Application Publication No. 2005-315125 (JP-A-2005-315125).
A clearance groove for the communicating hole processing between the adjoining crank chambers is formed in a top section of the partition wall. The communicating hole is formed by horning, from a bottom of the clearance groove, in a section of the partition wall that is integral with a partition wall between cylinders on the cylinder block side. Thereby, the partition wall around the communicating hole can be formed to have a thick wall and prevent stress concentration. Such a communication structure is described, for example, in Japanese Patent Application Publication No. 2007-321615 (JP-A-2007-321615).
Further, the communicating holes that have opening areas or opening shapes different from each other are formed in a plurality of partition walls between crank chambers of a plurality of cylinders. This allows obtainment of a sufficient opening area of the communicating holes and facilitates removal of cores during molding of the cylinder block. Such a communication structure is described, for example, in Japanese Patent Application Publication No. 2004-316556 (JP-A-2004-316556).
In the above-described crank chamber communication structure of a multi-cylinder internal combustion engine, if crank pin positions on the crankshaft are inverted at 180° between the adjoining cylinders as in an in-line four-cylinder engine, air may be moved back and forth between the adjoining crank chambers in a manner such that pressure fluctuation due to reciprocating motion of the pistons is relieved alternately between the adjoining crank chambers, thereby effectively reducing pumping loss. However, in multi-cylinder engines in which the relationship cannot be achieved such that the phases of the pistons are opposite between the adjoining cylinders, such as in-line multi-cylinder engines that have three cylinders, five cylinders, six or more cylinders and V-type engines, pumping loss cannot be certainly reduced.
Specifically, for example, if the pistons of a plurality of adjoining cylinders move in the same direction among the crank chambers corresponding to three or more cylinders and thus air flows toward one side in the cylinder arrangement direction through the communication hole, air is not released at the most downstream cylinder that is positioned at the end of the engine. At the cylinder, the piston pushes the air out to (pushes air back to) the adjoining cylinder against air flow. This results in pumping loss.
Further, in the multi-cylinder engines in which the relationship cannot be achieved such that the phases of the pistons are opposite between the adjoining cylinders, because there is no point that speeds of the pistons of the adjoining cylinders simultaneously becomes zero, the back-and-forth movement of the air itself becomes very complicated. Therefore, pumping loss is apt to occur not only at the cylinder that is positioned at the end but also at the other cylinders.